Most car radios are provided with a noise removal device for eliminating pulsating noises caused by an ignition plug of a car and entering in a radio transmission signal.
A noise removal device of this type is configured to detect pulsating noises in an audio signal by a noise detecting circuit and to switchingly remove the pulsating noises by a gate circuit.
FIG. 5 is a block diagram of a pulsating noise removal device used in most car radios.
In FIG. 5, a high-pass filter (HPF) 1 removes an audio signal S out of an input signal v.sub.i from a terminal 2 to extract a pulsating noise N alone. The pulsating noise N obtained from the filter 1 is amplified by an amplifier 3, and subsequently sent to an automatic gain control (AGC) circuit 4 and a monostable multivibrator circuit 5.
The monostable multivibrator circuit 5 produces a switching pulse in response to the pulsating noise N when the noise N is above a predetermined level, and shuts a gate circuit 6. In most cases, the gate circuit 6 is of a preholding type or of a signal compensation type. In this case, the gate circuit 6 is normally conductive to pass the input signal v.sub.i in the original form as an output signal v.sub.o through a terminal 7. When the switching pulse is applied from the monostable multivibrator circuit 5 to the gate circuit 6, the gate circut 6 is shut so as to not pass the input signal v.sub.i, and removes the pulsasting noise N involved in the input signal v.
The AGC circuit 4 controls the gain of the amplifier 3 to prevent continuous blockage of the gate circuit 6 and continuous interruption of the output signal.
FIG. 6 shows the waveform of the audio signal S from which the noise N is removed by the preholding type gate circuit.
In FIG. 6, S designates the audio signal (entered signal), and Tsw represents the switching time taken for removal of the pulsating noise N and having the time width corresponding to an estimated time width of a pulsating noise. T designates the period or interval of the pulsating noises which actually depends on noise sources. The switching is performed by the switching pulse from the monostable multivibrator circuit 5 of FIG. 5.
FIG. 7 shows the relationship between the audio signal frequency f.sub.s and the signal distortion Ds upon a switching. It is recognized that the distortion ratio increases with the signal frequency f.sub.s. A similar tendency is admitted when the gate circuit 6 is of a linear compensation type.
FIG. 8 shows an inversion phenomenon upon removal of pulsating noises by a switching. In the switching time Tsw, the audio signal S takes the form of P.sub.1 -P.sub.2 -P.sub.3 and loses a portion as much as an area As with respect to the original signal. When no noise removal is effected, a pulsating noise component An shown by hatched lines in FIG. 8 is added to the signal S. These components As and An both invite a distortion from the original signal. When component An is sufficiently large with respect to component As (when pulsating noise is large), the system is switched to effect its noise removal operation to reduce the signal distortion. Most noise removal devices are based on the foregoing configuration.
In contrast, when component An is small with respect to component As (when pulsating noise is small), switching the system rather invites an increase in the distortion, and invits a so-called "inversion phenomenon".
Therefore, noise removal operation must be withheld in this case.
However, since the area As of the signal lost by the switching changes with the signal frequency, that is, decreases with a decrease in the signal frequency and increases with an increase in same, it is preferable to perform noise removal operation at low signal frequencies and interrupt it at high frequencies causing an inversion phenomenon assuming that the magnitude of the pulsating noise is constant. However, prior art noise removal devices are not configured in this fashion.